Prototype headphone amp

The folks at Reference Media encouraged me to create a new series of lower priced, easier to assemble products that they can sell at their stores. This prompted me to re-consider my stance on printed circuit boards (PCB's). I still believe that they are not good for tube circuits because of the high-voltage and heat involved, and the capacitive properties of the board itself, but if designed and implemented with care they can work just fine. After all, most of the other tube manufacturers out there use PCB's. The new series will use PCB's for ease of construction, have simpler casework with silk screened front panels (instead of engraved) and integrated power supplies (instead of external).

The Sixty-Six Series (66-100 power amp, 66-001 preamp and derivatives) represent the premium products that we produce and will remain unchanged; full point to point wiring, external power supplies for the preamplifiers, engraved front panels, more robust and complex casework. I still believe that point to point is superior for tube circuits and sounds more transparent, and allows for optimum circuit component layout since it's three-dimensional. The lower priced series (yet to be named) will retain the same parts quality and circuit design but at lower price points for reasons stated above.

So, the first foray into this new paradigm is a headphone amp. The circuit is a virtual copy of the 66-001 line stage, minus input selector, balance, mute and mono controls. The original 66-001 preamp started out as a headphone amplifier that I breadboarded up several years ago and found to be an outstanding preamp. So it was a no-brainer to make a headphone amp out of this circuit.

The amplifier and power regulator circuits will live on the same PCB. This same circuit board can be adapted to a full blown linestage with some changes in casework and the addition of the requisite switches, pots and jacks. This allows flexibility and further reduces costs. It has stereo unbalanced inputs and parafeed transformer coupled outputs. There is a low/high gain switch that switches primary windings on the output transformer to accommodate differing impedances and sensitivities and should drive most any headphone out there. Each amplifier stage has multiple bypasses on the power rails, which is regulated, along with the filament supply.

Many hours were spent up front designing the circuit board, drafting the casework and specifying parts that will fit. This process resulted in me finding better parts that will be carried back over to the Series Sixty-Six products.

As with all PCB's I've designed, there were things that didn't quite fit, and a few mistakes. That's the way it goes; you never know until you get that first one and start loading parts. It's a process, but now all the corrections are made and the prototype is built. Not without a few mishaps! I had to replace the high-voltage regulator mosfet (4 different times!), because of shorting it out while measuring the rail voltage. The meter probe kept slipping off a resistor leg. Bang! One thing about PCB's- the parts are hard to remove once they're soldered in place.

In the photo you can see the two output transformers in front, the PCB mounted volume control and the large poly coupling caps. Further back are the tubes and related amplifier circuitry and at the rear of the board is the PSU regulator circuits. Outside the picture are the two power transformers. Of course this will all fit into a box about 12" wide by 10" deep and about 4" tall. The tubes will stick out the top of the box, which will have plenty of vent holes.

Playing back high-res flac files from my computer's external soundcard and driving my Grado SR80 'phones, the sound is wonderful, in fact, it sounds just like the reference 66-001 preamp upstairs in the living room..

Stay tuned for progress on this new product development. Next comes the casework and the revised PCB.

My visit to Reference Media

The Elliott Studio Arts 66-100 in a showroom!

I recently ran into my old associate Hans at a 7-Eleven. After a brief visit I found out that he works at the only high-end audio shop in Everett, WA; Reference Media (they also have a store in Bellingham, WA) http://www.reference-media.net/. I told him about my amp and preamp and we agreed on an audition of the 66-100 the store. I was surprised when I drove up and saw the store front, very nice. The inside is very well appointed with tasteful local art on the walls, a nice color scheme, wood floor and high ceilings. I was greeted by a McIntosh / Martin Logan system sounding sweet. The also stock Marantz, Peachtree Audio, Shunyata Research, Sonus Faber, Vienna Acoustics, Focal, ProJect turntables and more. In the picture below, up the stairs you can see the Golf Simulator system they sell set up for demo. Hans took a couple of swings to demonstrate, it's pretty impressive! You use real golf clubs and balls, driving them straight into a specially made screen. You can choose your course, the weather and wind conditions, etc. Neat. I got the grand tour, they have plans to install a dedicated theater room, build out some unused space for offices and divide up the main floor sales area into listening rooms. The store has a friendly atmosphere and the staff makes an effort to be helpful, informative and never snobby or pushy.

So, on to the main event. The first system we used to evaluate my amp was based on the Peachtree Audio iDecco as a preamp, driving the Focal Chorus 706V speakers. Cabling by XLO. Promising. Good sounding speakers, Hans said my amp tamed the tweeters somewhat, though you could still hear all the detail, and made them more enjoyable.

Next up on the same system were the Sonus Fabers, picture on the right. Not sure exactly which ones, probably from the Toy Collection. They were little towers w/ two 5" woofers and a tweeter. The sound was wonderful; rich, detailed with more bass than the last set of speakers, great imaging. But the best was yet to come with this setup. On the left is the Vienna Acoustics Mozart Grands being driven by the ESA 66-100. This was by far the best sounding speaker in this setup. Sublime.

OK, now on to the BIG system! McIntosh MCD500 CD player, McIntosh C2300 preamp and ESA 66-100 into the Martin Logan CLX's, with the Shunyata Hydra power conditioner. WOW! And especially wow when the volume was raised up. The amp drove them perfectly, taking control of the huge bass panels like nobodys business. The midrange had that tube realism and the highs were perfectly rendered. A very good match! It certainly helps having a good source and preamp to show the full potential of this amplifier.

All in all an enjoyable visit and a great opportunity to see one of my designs mated with some high-end speakers and equipment, and to confirm that it is indeed in good company - a true high-end product. And a great store to experience the high-end in comfort!

When I retuned home after exposure to all that high-end gear I was not dissapointed with the sound of my own system, consisting of ESA 66-001p Preamp, ESA 66-100 power amp (prototype), Rega Apollo CD player (modified), Thorens TD125 w/ MMF 5 tonearm and Klipsch RF83 speakers (modified). Listening to Pink Floyds incredibly detailed and layered recording, Momentary Lapse of Reason, on vinyl was re-affirmation that I have, indeed, achieved high-end sound in my own home.

Now that's what this is all about!

Birth of an Amplifier

Here's a short description of the construction of an Elliott Studio Arts 66-100 power amplifier. This is a basic model, unbalanced and without a triode/UL switch (which switches the output stage between modes). The first step in construction is to assemble the chassis and related components. Shown below is the top panel to which the circuit board is installed after it is seperatly constructed, as well as the power and output transformers. A horizontal brace can be seen, situated below the heavy transformers to provide additional strength, and a lightweight vibration damping material is applied to damp rining. The tubes, bias pots and switch protrude through the holes and the bias meter occupies the rectangluar opening in the center.

Next you can see the bottom of the chassis with the seperate front and rear sub-panels installed. Seperate sub panels facilitate changing the configuration without having to re tool the entire enclosure. In example, the rear sub panel for the balanced version of the amplifier is different but is the only part that needs to be changed. This makes the product flexible and economical to build in small quantaties. Also seen is the cooling fan, input and output jacks, power inlet / filter and fuses. Up front you see the main filter caps, smoothing choke and power switch. In the center is the terminal block where all the power wiring will land when the top panel and circuit board are installed.

The next picture shows the circuit board with the tube sockets, terminal strips, bias meter & pots, and ground wiring installed. This "circuit board" is actually 16ga. steel which provides a rigid backbone for the circuit to live and an active ground-plane with good RFI rejection. All the components are riveted directly to the circuit board and will not vibrate loose over time. All the wiring is point to point just like the old days. This method of wiring is time and labor intensive but is more robust, superior for the high-heat and high-voltage environment of tube amplifiers, and sounds better. I have repaired some contemporary tube amps that use printed circuit boards and have seen catstophic failure of the board itself where the heat of a power resistor actually burns a hole through it. No way to fix that without replacing the entire board! Printed circuit boards also exhibit small amounts of capacitance between traces and the board material itself is not a very good dielectric, which can smear the sound to a degree. That said, we do use printed circuit boards in non-critical areas, like power supplies and logic circuits.

Here things are beginning to come together. This is part of the on-board power supply bypassing for the output stage. Also seen in the background is the filament supply circuit with damping resistors and +50V injection to cap off hum and prevent too high a voltage differential between cathode and filament at the input stage phase splitter. Heat resistant ceramic tube sockets are employed for long life and no arcing. All signal path junctions are soldered with silver content solder and care is taken at every step to ensure solid connections and future serviceability.

The next picture shows the input stage under construction. Each half of the input tube (12au7a) is bypassed to prevent power supply interaction between stages. The first stage is a simple voltage amplifer with feedback from the output transformer injected at the cathode, which is bypassed with high-quality solid polymer electrolytic caps. These have outstanding low impedance and ESR properties and live well in high-heat environments. This stage is direct coupled to a spit-load phase splitter which has a build-out resistor in the cathode leg to match output impedances of both halves for better high-frequency linearity.

In this picture the input stage / phase splitter and driver stage are completed. In a novel implementation of local feedback and bootstrapping this little stage delivers the needed voltage swing to drive the output stage to full power at low distortion. One wouldn't normally consider the 12au7a to have the balls to do this! High-quality polypropelyne coupling and bypass capacitors are used throughout. All wiring is high-tempature and rated for applied voltage.

Here's the completed circuit board installed to the top panel with transformers mounted and wired into the power supply circuitry, which can be seen along the bottom. On the left is the main B+ supply, on the right is the bias supply and filament damping circuit. Off to the side can be seen a small printed circuit board which houses the 60 second B+ delay, which allows the filaments to heat up fully and applies the bias voltage before the high-voltage hits the tubes. The output stage employs an LR network to equalize the small inductance differences between windings in the output transformers. This also improves high-frequency linearity.

Speaking of transformers, all transformers used in Elliott Studio Arts products are custom wound to our own specifications by a well respected company based in New Mexico, Edcor Electronics. The excellent sound quality and bass control of these amps can in large part be credited to these transformers. The power supply transformer is massive with ample power regulation; in conjunction with the big computer grade caps and choke provide huge power reserves which greatly increase dynamic impact and slam. The power supply is very clean and well protected with fuses and inrush current limiters, which can be seen below mounted to the terminal strip in the bottom chassis.

The top cover and circuit is now wired to the bottom chassis, the input connections have been wired and the output transformer secondaries are hooked to the speaker jacks.

The next step is to install tubes, connect meters to monitor B+, bias and filament voltages and an O'scope and dummy loads to the output. Now fire her up!

After everything checks out, the bias is increased until the proper current reading is applied to each tube, voltages are checked again. Once everything stabilizes it's time to throw some sine waves at it and check for oscillation, frequency response and max power output. Then a complete battery of computer generated tests including frequency sweeps, distortion and square wave response is done and documented for record keeping.

The face plates are installed after all the testing is complete. The amp is then installed in my reference system for a week to burn it in and listen for any anomalies. It is then packaged up and shipped to the customer for many years of happy listening!

Tube Stomp Box

Tube Stomp Box for digital recording


A good friend of mine is an accomplished guitarist who has a recording setup for his band. Part of his rig is an amp simulator plug-in that he uses with digital input (AD converter) to simulate the tone of an amplifier. Recently he asked if a tube guitar pedal would add that tube 'roundness' to enhance the sound of his recordings. This sparked an idea..















I figured that any stomp box has to be small and portable, powered via wall-wart and have a bypass switch. Having lots of tube stuff and parts around I scrapped a prototype together on the bench. In the photos above and below you can see the first attempt at the circuit. To fit the requirement of wall-wart power meant that a high-voltage B+ supply had to be derived from a low voltage source. I settled on a 24VAC @ 1 amp supply sourced from Triad, which when run into a voltage quadrupler yields 160VDC at about 200 ma. The filament supply would have to come from the 24VAC, padded down with series resistance to get 12VAC. OK, so with the PSU figured out it was on to the tube gain stage.

















My first attempt was to use the venerable 12AX7, which as you will see proved problematic. I used both halves of the tube to make two identical stages with a gain of about 35dB each. I put a volume control between stages figuring that the guitar volume control could be used a a gain and the stomp box control could be used for drive so you could get some overdrive. The gain of the whole circuit was way too high to input into a guitar amp (or amp simulator) so the plan was to knock it down with an output transformer.

Alright, so far so good.. looked clean on a scope, got plenty of gain and overload margin was acceptable. Then I put it in a box and hooked it to an amplifier- BUZZ! HUUUMM! Oscillation! Microphonics! High frequency rollof! I was surprised, everything looked fine before putting it in the box. Too much gain coupled with wiring capacitance and high impedance created all sorts of problems. I worked on adding more filtration to the power supply, DC filaments and shielded cable to no avail (high-gain, high impedance tubes such as the 12AX7 have issues like this).
I had to look for another solution.
















Well, of course! Change the tube to a 12AU7, get rid of all that gain and all the associated problems go away too. And as fate would have it, the plate and cathode resistor values worked out perfectly for the given voltages so the 12AU7 could just drop right in. Less gain equals lower miller capacitance, meaning flatter frequency response and less sensitivity to induced hum, microphonics and oscillation. This tube also has lower output impedance making it easier to drive long(ish) unbalanced interconnects which are likely to be use with this device. So with less gain and lower output impedance the output transformer could be eliminated too.

















So, done for now. I will ship this out to him to see what he thinks, probably end up making some changes according to his feedback and then who knows? Maybe another Elliott Studio Arts product?? We'll see..

 Nope! This project was a failure... My friend could not get rid of the buzz. Probably due to sharing the same supply for the filaments and the B+. He still has it so I have no way to experiment and figure it out~ Oh well, next time I'll do an isolated DC supply on the filaments..

This really makes a difference....

I am amazed at what I'm (not) hearing!

Have you ever noticed how your system sounds great on some days and kind of grainy and harsh on others? Usually sounding more sweet late at night? Dirty power - that's what I say! During the day when usage is high in your area microwaves, electric motors, ubiquitous switching supplies, fluorescent CFL bulbs, etc. generate tons of spurious RF and harmonic noise that gets into your sound system and robs it of clarity and silence, adding grit and grain and listener fatigue.

I discovered the Surgex power conditioner (http://www.surgex.com/) years ago when their product rep visited our office for the dog and pony show, and since then always wanted one. I have recommended them to clients (see my previous post: "Power Conditioners and Hype") and have heard how they cut down on the noise in various sound and video systems, not to mention the superior surge protection they afford. So I finally broke down and made the purchase, opting for the more industrial looking (and cheaper) pro A/V unit rather then the home model.

Since all my equipment was plugged into a power strip it was a simple install: Plug in the Surgex and plug the power strip into it. I let it cook for a day before I had the opportunity to really evaluate my system with this new change. My first impression was immediate- I put on my vinyl copy of Alan Parsons Project 'Pyramid'; it sounded as it does late at night when the backgrounds are blacker, the highs are more shimmery and pure and the sound is generally more liquid and transparent. But even more so than what I'm accustomed to hearing in those (rare) late night sessions: blacker silence, more transparent, more vivid, more liquid. No fatigue, zero, nada, nyet. The bass tightened up, cymbals sounded more real and brassy, voices more pure. I heard vocal tracks way back in the mix I hadn't heard before. Even the hum in my already quiet phono stage w/ no signal was less.

The improvement to CD's was more profound, making them sound more analog and pure, removing that last bit of elusive to describe 'digititus', especially in the high frequencies; again, cymbals sounded more like cymbals. Everything was more liquid, more transparent, more like music from top to bottom. More detail but without the fatigue. I could go on but I'm starting to sound like one of those audiophile magazine reviewers (which I read all the time!)


The bottom line is if you've invested substantial time and resources into your system you owe yourself the protection that these units afford. The cost of the Surgex isn't cheap but not much compared the the price of replacing all your gear, some of which may not be replaceable. But with the improvement to the sound, having the best surge protection technology in the world is just the icing on the cake! You cannot buy these units retail at the Best Buy, you'll have to go to the website and find out who your local rep is and order one from them, but it is mandatory!

Happy Listening!

The making of a preamp

Building the 66-001p Vacuum Tube Reference Preamp
(an overview)

Well, it's been a while since I've posted here. I've been working on a preamp that is bound for review. The picture above is the regulator section for the outboard PSU, which in this case is configured for 120VAC / 60Hz. The regulator board is the only circuit board used in the entire product. Circuit boards are convenient but can color the sound as they are a giant capacitor with not so good dielectric. All of my products use point to point wiring as seen in this photo of the RIAA section under construction.

The circuit resides on an aluminum sub-chassis that is isolated from the main casework via 8 nitrile rubber bushings. The entire sub-chassis is the ground plane, and there is copper buss wire connecting all grounded points in the circuit. This provides a low-impedance ground path and helps to shield against noise. The RIAA preamp uses a fet/triode cascode for the input stage for high gain and low capacitance.

All the wiring is high-temp Teflon insulated solid conductor for signal and stranded for power. Silver solder is used throughout along with close tolerance metal film and wire-wound power resistors, polypropylene capacitors, (hand matched polypropylene and polystyrene caps used in the RIAA filter), and high-quality electrolytic and polyester caps for power supply bypassing. Solid polymer electrolytics are used for the cathode bypass capacitors; these have very low ESR and a long life span. You can see them in the picture above zip-tied to the yellow poly bypass caps. Below shows each tube section has 110uF of bypass capacitance. High quality ceramic tube sockets are used for longevity.

Below you can see things starting to come together. Those are the output transformers, used here for gain reduction and impedance matching. These transformers have an output impedance of 8 ohms! They drive headphones with authority and since the output impedance is so low you can run very long interconnects without high frequency roll off. The signal cable you see is high-speed digital coax used for HDTV and VGA applications. It has very low capacitance and uses foamed Teflon for the dielectric, has a silver plated solid copper center conductor and dual shield (foil and braid).

The front panel is where you see the functional interfaces; volume control (Alps Blue), balance, mute and mono, input selector (silver contact) and headphone jack. The balance control works in shunt mode and is out of the direct signal path. Moreover, it's action is limited to about 10dB of variation. The balance, mono, volume and mute controls can be seen in this picture.

The casework is steel with a nice satin texture paint job which hides fingerprints and other maladies well. The steel does a good job blocking RF interference.

Below is the nearly completed preamp awaiting the top and bottom covers and faceplate. These units take about 24 hours of labor to put together as every component has to be placed, every wire cut to size and dressed, components have to be measured and matched. Then multiple tests and measurements are done and finally a listening evaluation. This is not to mention the amount of engineering, prototyping, testing, measuring, listening, re-evaluating and modifications that went into the design. But a labor of love, to be sure!

So, I'm not going to give away all my secrets~ But I can assure you that this preamp performs very well in the measurement world and even better in the listening room! For full specifications and performance data please visit my website:

http://elliottstudioarts.com/

Resurrecting the McIntosh MC-30

The Mighty McIntosh




My boss Jack has had a pair of old dusty and slightly pitted MC-30's laying in his office for several years. The story was that they belonged to his father who purchase them new sometime in the golden age of Hi-Fi, and they had been languishing at the home of one of his other sons for years. Jack came across them and rescued the pair thinking that someday he would put them to use driving a pair of Renkus-Heinz commercial 16ohm speakers we've had in our warehouse, also collecting dust. Well, he finally asked me if I could do something with them.. duh!

I got them home (heavy buggers!) and got one up on the bench, opened it up and found that some repairman from the distant past had done some meatball surgery that had to be corrected. One of the amps power supply section was really messed up by someone - the rectifier feeds a 30uf cap first, then a 150 ohm power resistor to an 80uf section forming a CRC filter. But what I found was actually a 10K power resistor feeding the second stage filter (which is B+ for the output stage). I don't see how this could have even worked! Plus, there was a 12au7 instead of a 12ax7 in the driver cathode follower stage. Also, the power cord had long since disintegrated and there were some burnt wires (why did they use 22 gauge wires from the rectifier to the filter cap??), but overall things looked pretty good.

First things first, get rid of all the 45 year old electrolytics! Except for the main filter caps, which measured good with the cap meter (all 3 sections), there were dried up bias supply caps and input stage cathode bypasses that had to go. Also, the paper/wax coupling caps were suspect; I've heard that they tend to get leaky- though some audiophiles like the way they sound. I don't. I like detailed music reproduction, thank you. These caps would be good for a guitar amp where you're going for a colored sound, not Hi-Fi (now days there are very expensive 'boutique' paper in oil capacitors available, I'm not talking about these). Most of the other coupling caps are some sort of plastic encapsulated film, not sure about them. I left them in for now.

Since Jack is not an audiophile I didn't use any exotic parts; rather than an 'upgrade' this project is more of a resurrection. I used what I had on hand. I replaced the caps listed above with standard modern electrolytics, replaced the output stage coupling caps with poly film (.47uf @ 630V) and installed grounded power cords (I cut the IEC ends off of some stock power cords that come with AV equipment). I also put in some 330uf @ 450V Panasonic snap-in caps that I've had laying around since the '80's (these were in the first iteration of the ESA 66-100 which have long since been scrapped for parts). These I paralleled with the 80uf main B+ caps, and bypassed them with some .1uf 450V poly's.

I do see alot of room for improvement, though. Like replace all the old coupling caps with decent film caps like Solen or better, do more power supply bypassing for the input and inverter stages and get rid of all the unnecessary input wiring and hardware. The input RCA's could use an upgrade, and a heavier barrier strip so you can connect some 'real' speaker cables would be nice.

The design of the amp is interesting. There's the obvious 'Unity Coupled' output transformers which have a cathode winding from one tube in phase with the plate winding of the other tube. There is the 'bootstrapping' of the driver; the B+ for the driver load resistor is derived from the plate winding of the same phase tube giving a positive feedback to the driver tube while delivering negative feedback to the output tube. And there's the 12ax7 cathode follower between the driver and output stages, direct coupled to the input grids and therefore having the -45V bias voltage on their cathodes. This tube is also bootstrapped; the positive feedback is necessary to derive the very high AC drive voltage required for the output stage (remember the cathodes are in the transformer winding causing negative feedback, lowering the overall gain of the stage). The input stage is pretty standard and uses 1/2 of a 12ax7 direct coupled to a 12au7 configured as a cathode coupled phase inverter. The McIntosh design was way ahead of its time and the performance is reported to be excellent. All in all, alot of circuitry for 30 watts!

Ok, so after all the circuit work, I cleaned up the chassis and tubes, plugged them into AC, and checked out the voltages. One amp measured spot-on, the other had some funkyness in the output stage. One output tube was in cutoff and it's bias voltage measured -220v! Some visual inspection revealed that I had inadvertently cut a 220K cathode resistor for one of the followers, which supplies bias voltage to the grid of the 6L6. Repairing this error fixed it right up!

Alright! I'm too impatient to put these things on the scope and do the requisite battery of tests, I just want to plug them in and play some tunes! They sound pretty fine, I must say. They're a little dark and the bass is not as tight as I'm used to (the 66-100's are highly resolving and have excellent, tight bass). But overall, they have that tube magic. I bet new output tubes and performing the rest of the upgrades as listed above would bring them into better focus, but my boss just wants them to work and I think that they'll do just fine with the high-sensitivity horn loaded Renkus-Heinz speakers he's going to drive for his office system.

I promise I'll get these things hooked up to the scope and see what the power, frequency response and square wave look like, but for now I'll just enjoy them before I have to give them back!

UPDATE: 3/28/10

Just put these babys on the test bench. Amplifiers terminated into 8 ohm resistive dummy loads.

Square wave response is good for 1Khz, 10Khz is a bit rounded off on the leading edges, suggesting a bit of high-freq. rolloff.

Freq. Response is flat from about 30hz out to about 15Khz, and there's a bit of droop on either extreme, as viewed on an o-scope.

Power is 45 watts before visible clipping on the amp with the better output tubes. The one w/ the weaker set puts out about 25 watts before clip. Think I'll recommend new tubes to Jack.

No distortion measurements made at this time.

I had fun restoring these amplifiers. They are very well made, well laid out and easy to work on. I'm sure Jack will enjoy listening to them as much as I have!